Dig up anyone who's built a Long-EZ, Vari-EZ, Quickie, Q2, Q200, or other all foam, full scale, person carrying, experimental aircraft. They build 16 to 30 foot wings from 4 foot long hotwired cores. There is a standard technique for joining that results in a strong light joint.

The brief description is:

1. A good butt joint of the foam, flat surfaces, no more than 1/16" gap anywhere.

2. A medium to dry slurry of microballons and epoxy in the joint, carefully portioned and applied so that it fills the joint but does not slop out any "edges".

There is a tiny bit more to it than described above, but that's the basics. Any set of plans and/or builders guide for a full scale "moldless composite" (i.e. Rutan style) aircraft will have all the details on how to join cores.